Any electronic component needs to be contacted electrically for its operation; in some cases, moreover, it may be preferable that this contacting should occur in an elastic way.
A typical example is that of sockets that are used for contacting electronic components during their test at the package level; another example is that of probe cards, which are used for contacting electronic components during their test at the wafer level. In both cases, the electronic components to be tested are contacted in an elastic way to avoid their damaging. An elastic structure may also be required to electrically connect a chip of semiconductor material (wherein an electronic component is integrated) to a circuitized substrate of a corresponding package—for example, in the flip-chip technique—in order to reduce mechanical stresses between the chip and the substrate. The same necessity may occur when it is required to electrically connect different electronic components in a Multi-Chip Module (MCM), and in power components on DCB (Direct Copper Bonding) plates.
For this purpose, several electrical contact devices (or simply contacts) of the elastic type have been proposed.
Particularly, contacts being known as “pogo-pins” are commonly used in the sockets and in the probe cards. In general, the pogo-pins are formed by metallic springs or polymeric elements that are made conductive by embedding suitable metallic particles—for example, as described in document US-A-2006/0145719 (the entire disclosure of which is herein incorporated by reference).
However, the spring-based contacts are relatively cumbersome and therefore they do not allow obtaining structures with reduced pitch. Indeed, the springs have a minimum working length, being defined by the maximum deformation that allows maintaining their elastic properties, which is rather limited (for example, hardly higher than 60% of their resting length); therefore, the springs should be maintained relatively long to allow a deformation that provides acceptable values of a reaction force in response to a corresponding compression force. Moreover, the springs exhibit a direct proportionality between the (compression/reaction) force and the deformation; this may be a drawback in some applications (for example, in the probe cards and in the sockets). Indeed, in order to obtain a correct electrical contact with the electronic components (of the wafer or of the package) to be tested, each contact should provide a nominal reaction force being due by a corresponding nominal deformation. Generally, the probe cards and the sockets include a very high number of contacts (of the order of some thousands), so that it is not possible to ensure that all the contacts reach the wafer or the package, respectively, at the same time (because of an unavoidable planarity error). Consequently, it may be necessary to apply a compression force that allows moving the contacts closer by a distance that is higher than the one being needed to obtain the nominal deformation at least of the expected planarity error (in order to guarantee that all the contacts undergo the nominal deformation). However, the proportionality between the deformation and the force causes the reaction force being exerted by the first contacts that reach the nominal deformation to be remarkably increased (because of their additional deformation), with the risk of possible damages to such contacts; moreover, this may require a significant increase of the compression force to be applied to the probe cards or to the sockets (in order to overcome the additional reaction force being exerted by the contacts that first reach the wafer or the package, respectively).
The conductive polymer-based contacts, instead, have a reduced working life (because of the structural modifications that are caused in the polymer by the added metallic particles); in any case, the need of adding high amounts of metallic particles to the polymer for obtaining an acceptable conductivity (even higher than 50% in volume) considerably limits the possibility of controlling its mechanical characteristics—thereby making very difficult, if not impossible, to obtain the desired elasticity.
The same drawbacks pointed out above for the spring-based contacts are also suffered by the structure being described in document U.S. Pat. No. 6,796,810 (the entire disclosure of which is herein incorporated by reference). Indeed, in this case there is proposed to use elastic conductive columns, which are surrounded by an insulating material.
Alternatively, document US-A-2002/0086566 (the entire disclosure of which is herein incorporated by reference) proposes using an elastic polymeric element that embeds an electric conductor at the liquid state. However, this structure is difficult to make, requires non-standard dedicated processes, and it is limited by the need of using conductive materials that are liquid at the normal working temperatures of the contacts.
Another possibility is described in document U.S. Pat. No. 4,820,376 (the entire disclosure of which is herein incorporated by reference), which proposes embedding a column of conductive particles into an elastic polymeric element. A drawback of this structure is that the electrical conductivity is not fixed, but it changes with the compression force that is applied thereto.
A different contact structure for probe cards is described in document U.S. Pat. No. 5,367,254 (the entire disclosure of which is herein incorporated by reference). This document proposes using a conductive wire, which is housed within a rigid guide with recesses that are staggered to each other longitudinally (with every pair of adjacent recesses that are arranged in radially opposed positions); the wire is free to slide along the guide, with an end that is fastened to the guide and another end that protrudes therefrom. In this way, when a load is applied to the protruding end of the wire, it buckles (collapses) thereby re-entering the guide; the proposed structure of the guide allows controlling the deformation of the wire, so as to ensure that it returns to its original position when the load is removed. However, the electrical contact so obtained is remarkably unstable.
Another structure being based on the buckling of conductive wires is described in document WOA-95/04447 (the entire disclosure of which is herein incorporated by reference). In this case, the buckling conductive wires transversally deform an elastic polymeric element (which maintains the conductive wires insulated, and controls their deformation by means of rigid inserts). In this way, the axial reaction force is provided by the conductive wires, while the polymeric element does not undergo any axial compression (because of the presence of a corresponding rigid mechanical constraint). Therefore, the conductive wires are subject to a remarkable mechanical fatigue (with the same document that cites an alternative wherein it is possible to replace the conductive wires that break because of the undergone fatigue).
In the packages, instead, contacts of the elastic type between the chip and the substrate may be made with compliant bumps, wherein the bumps are mounted on micro-springs. This structure is however relatively complex and cumbersome, and it suffers from the same drawbacks being pointed out above for the spring-based pogo-pins. Alternatively, contacts of the rigid type are often used; in this case, once the chip has been soldered to the substrate, an elastic under-filling layer is then added between the substrate and the chip.
Another contact structure of the elastic type (for use in sockets, probe cards, packages and multi-chip modules) is described in document WO-A-2006/066620 (the entire disclosure of which is herein incorporated by reference). In this case, the contact is formed by an elastic polymeric core (being either conductive or insulating), which is coated by a metallic cover with a pyramidal or truncated pyramidal shape.
Conductive wire-based contacts are also known; this structure is generally used to make elastic interposers, which are formed by a polymeric layer that sustains wires being embedded therein—for example, as described in document U.S. Pat. No. 5,334,029 (the entire disclosure of which is herein incorporated by reference).
Another example of interposer is described in document U.S. Pat. No. 4,998,885 (the entire disclosure of which is herein incorporated by reference), with a similar layer that mechanically sustains conductive wires thereby insulating them electrically. In this case, notches are made in the polymeric layer between the different conductive wires, which notches are used (according to their size and spacing) to control the compression uniformity of the structure.
The document U.S. Pat. No. 6,264,476 (the entire disclosure of which is herein incorporated by reference) describes a different type of interposer, wherein each contact includes a conductive element being formed by a spring (which contributes to its elasticity in synergy with a corresponding elastic element) or by wires too fine to stand on their own.
Moreover, a particular technique for making contacts that are based on the raising of wires being made on a substrate of porous silicon, which wires are then embedded into a polymeric layer, is described in document WO-A-2007/104799 (the entire disclosure of which is herein incorporated by reference).
Other types of contacts are described in the document U.S. Pat. No. 6,079,987 (wherein the conductive element is formed by twisted wires), in the document US-A-2002/060579 (wherein the conductive element is obtained by a wire being fed from a reel that is coupled with a spring), in the document WO-A-98/41877 (wherein the conductive element is formed by a plunger connected to a tubular spring), and in the document U.S. Pat. No. 5,174,763 (wherein the conductive element is formed by two probes with a telescopic structure that are coupled with a spring or a tube of rubber)—the entire disclosures of which are herein incorporated by reference.